1. Field of the Invention
This invention relates generally to a separation system and process to obtain a primary separation of particulate solids from a mixed phase gas-solid stream in a Thermal Regenerative Cracking (TRC) apparatus and process described in U.S. Pat. No. 4,061,562 to McKinney et al and U.S. Pat. No. 4,097,363 to McKinney et al.
2. Description of the Prior Art
Chemical reaction systems utilizing solids in contact with a gaseous or vaporized stream have long been employed. The solids may participate in the reaction as catalyst, provide heat required for an endothermic reaction, or both. Alternatively the solids may provide a heat sink in the case of an exothermic reaction. Fluidized bed reactors have substantial advantages, most notably an isothermal temperature profile. However, as residence time decreases the fluidized bed depth becomes shallower and increasingly unstable. For this reason tubular reactors employing solid-gas contact in pneumatic flow have been used and with great success, particularly in the catalytic cracking of hydrocarbons to produce gasolines where reaction residence times are between 2 and 5 seconds.
As residence times become lower, generally below 2 seconds and specifically below 1 second, the ability to separate the gaseous products from the solids is diminished because there is insufficient time to do so effectively. This occurs because the residence time requirements of separation means such as cyclones begin to represent a disproportionate fraction of the allowable reactor residence time. The problem is acute in reaction systems such as thermal cracking of hydrocarbons to produce olefins and catalytic cracking to produce gasoline using improved catalysts where the total reactor residence time is between 0.2 and 1.0 seconds. In these reaction systems conventional separation devices may consume more than 35% of the allowable contact time between the two phases resulting in product degradation, coke formation, low yields and varying severity.
In non-catalytic, temperature dependent endothermic reactions, rather than separating the phases, it is possible to quench the entire product stream after the requisite reaction period. However, these solids are usually recycled and are regenerated by heating to high temperatures. A quench of the reactor effluent prior to separation would be thermally inefficient. However, it is economically viable to make a primary separation of the particulate solids before quench of the gaseous stream. The residual solids in the quenched stream may then be separated in a conventional separator inasmuch as solids gas contact is no longer a concern.
In some reaction systems, specifically catalytic reactions at low or moderate temperatures, quench of the product gas is undesirable from a process standpoint. In other cases the quench is ineffective in terminating the reaction. Thus, these reaction systems require immediate separation of the phases to remove catalyst from the gas phase. Once the catalyst has been removed, the mechanism for reaction is no longer present.
The prior art has attempted to separate the phases rapidly by use of centrifugal force or deflection means, as exemplified by U.S. Pat. No. 2,737,479 to Nicholson; U.S. Pat. No. 2,878,891 to Ross; and U.S. Pat. No. 3,074,878 to Pappas.
In a TRC system having a short residence time (i.e., in the range of 0.05 to 2 seconds, at temperatures in the range of 1300.degree. and 2500.degree. F.), the product of C.sub.2 H.sub.4 is favored. This means that the reaction must be quenched rapidly. When solids are used, they must be separated from the gas rapidly or quenched with the gas. If the gases and solids are not separated rapidly (but separated) as in a cyclone, and then quenched, product degradation will occur. If the total mix is quenched, to avoid rapid separation, a high thermal inefficiency will result since all the heat of the solids will be rejected to some lower level heat recovery. Hence, a rapid high efficiency separator is optimal for a TRC process.